“There are hundreds of strains of cold viruses, and they have different rates of infectivity,” said Dr. Jacobson, an internist and allergist. “Previous exposure to the virus at hand should lead to immunity, but immunity can wane over time, and viruses mutate as well.”

Resistance can be compromised by an array of factors, from sleep deprivation to underlying respiratory diseases, she said. Immunodeficiency, whether caused by medication or disease, can also predispose a person to catch cold.

It is possible to have a cold without noticeable symptoms, which vary from strain to strain and person to person, Dr. Jacobson said. Communicability also depends on the amount of the virus in the cold carrier’s secretions. It is higher earlier in the cold and begins to drop after three or four days.